Pre-loading of building sites over compressible strata

ABSTRACT

A method and associated apparatus are given for a better approach to the pre-loading technique currently used to improving otherwise unsatisfactory ground conditions so that a site may be used to support an engineered structure or building. The method allows for pre-loading of a site to proceed where there are preexisting buildings on adjoining properties without causing damage to, or interference with, those properties while at the same time allowing full utilization of the building area. This involves using sections of sheet piling to isolate and contain ground movements in the subject site from having any influence on adjacent land, thus preventing damage to neighboring property. Apparatus to effectively support minimal lengths of sheet piling by means of a novel adjustable anchoring system is given. Apparatus for a novel monitoring system to provide comprehensive data on the progress of ground improvement is also given. Both pieces of apparatus are fully recoverable upon completion of pre-loading and are then reusable for similar future work.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C 119(e) to United States Provisional Patent application No. 61/124,886 filed Apr. 21, 2008, the disclosure of which is incorporated herein by reference.

BACKGROUND OF INVENTION

1. Field of Invention

The method and apparatus described herein are intended to provide the means to successfully prepare, or precondition, a parcel of land in an area of otherwise unsatisfactory geotechnical ground conditions so that it may be improved for optimal development and use as a building site, without causing damage to, or interference with, adjoining properties. The approach is such that essentially all hardware necessary for the accomplishment of the work is recoverable and may be used again for subsequent similar projects; thus, the hardware is a component which might be obtained for temporary use from one of the equipment rental outlets commonly used by building contractors.

2. Description of Related Art

Current practice, which is a well known approach, is properly referred to as surcharge-pre-loading, but following common usage will be abbreviated to “pre-load(ing)”. It involves placing a mound of mineral fill, such as sand, over the part of the site on which it is intended to construct a structure; for ease of presentation a simple building is used hereafter for illustrative purposes. The mineral fill (pre-load) covers the footprint of the building as well as a perimeter zone around the building's circumference. The width of the perimeter zone is based on the extent of the side slopes the fill mound adopts, as well as any additional space the engineering designer may deem desirable as a safety margin. The height of the mound is chosen so that the sand fill will impose a base pressure on the foundation strata which is greater than the pressure the proposed building will subsequently impose on those soils after the fill is removed. Usually the fill height is selected to be equivalent to at least 1½ (one and a half) times the design structural pressures. In this way the foundation soils are preconditioned to carry the structural loads without further settlements thereby ensuring the building will not suffer subsequent deformations leading to structural damage.

3. Problems with Current Practice

The problems which are a consequence of the way things are done at present are illustrated by means of the sketches shown in FIG. 1. The four rows of illustrations show the sequence of events. The top row shows the situation before anything is done to alter the existing conditions on the site. The second row shows the pre-load in place. The third row depicts the ground configuration (vertical scale amplified for illustrative purposes) at the end of ground treatment. The bottom row shows the new structure in place.

In order to optimize the size of the usable area of the building site, the fill mound is placed as close to the adjoining property as possible, that is, with the toe of the side slope reaching the property line. In some cases, matters are made more critical by building a temporary retaining wall (typically made of Lock-Blocks) to bring the top surface of the fill as close as possible to the property line by abbreviating the side slope. In many cases, depending on the nature and thickness of the native ground, this approach, because of deep-seated settlements of the native ground, can lead to serious damage to adjacent preexisting buildings. This situation is illustrated on the LEFT side of FIG. 1 as Option A.

Alternatively, as shown on the RIGHT side of FIG. 1 as Option B, the designer will anticipate the potential for damage to the adjoining site and leave an adequate margin between the proposed structural footprint and the property line. This will ensure the settlement profile induced by the pre-load fill will remain substantially within the proposed building site and not impact the neighboring property. This proper design approach has, of course, the consequence of cutting back on the area of the subject property which can be developed for use as building sites, and results in narrow permissible building width(s), as depicted in the sketch.

SUMMARY OF THE INVENTION DISCLOSURE OF THE INVENTION

The novel method suggested here is to completely isolate the ground of the adjoining properties from the compressible ground beneath the building site, and thereby ensure that site preparation work on the subject site will have no influence outside its own property boundary. In addition, two pieces of novel apparatus are proposed to: enable the method to work effectively as intended; and, to monitor the progress of ground improvement to clearly identify the moment when ground treatment has achieved its purpose so that the hardware can be removed without unnecessary delay.

The method of isolating the building site from adjoining properties is by cutting through the full thickness of the compressible ground immediately beside the property line. This can be accomplished by temporarily installing a sheet pile wall penetrating to, and founded somewhat below, the interface between the compressible strata and underlying more competent stratum. At the end of the process the sheet piling is fully recoverable and reusable. This sequence of events is illustrated by the by means of the sketches shown in FIG. 2.

By laterally restraining the sheet pile wall using the novel anchoring system advocated herein, the depth to which the piling need penetrate the competent stratum is kept to a minimum. Keeping pile penetration to a minimum in the competent stratum will lessen any impact on the neighboring properties, such as vibrations experienced while driving the piles into the ground. The anchoring system is such that it can take advantage of, and is adaptable to, deformations of the pre-load fill mound during the progress of ground treatment. At the end of the process the anchoring system is fully recoverable and reusable.

By observing the progress of treatment using one, or preferably, an array of the novel monitoring posts described herein the optimum period of treatment can be reliably determined, so that the pre-load fill can be removed and construction of the building proceed at the earliest moment. At the end of the process the monitoring posts are fully recoverable and reusable.

Therefore, by means of the method and apparatus advocated herein, building sites can be developed to their maximum areal extent within a carefully delineated time period, while avoiding damage to neighboring property. This is accomplished using hardware which is fully recoverable and reusable, and therefore, could be made available to smaller contractors on a rental basis, that is, without them having to invest heavily in specialized equipment.

OBJECTS AND ADVANTAGES

This approach whereby sheet piling is used is illustrated in FIG. 2. The resulting benefits can be seen in comparison with the less economically desirable geometric arrangements shown on FIG. 1.

The use of the novel monitoring posts makes for more comprehensive, more frequent, and more cost effective data acquisition. Obviously, it is possible to feed these data into a digital recorder, and furthermore, to transmit that data by modem (or similar technology) to a design office, making it possible to have instant and continual information on the behavior of the ground treatment progress. On the basis of this stream of data, apart from informing the engineer when the pre-load had done its work in preconditioning the compressible strata and the pre-load fill can be removed, it would allow the contractor to anticipate the completion date. The project management is thereby enabled to maintain a tight schedule.

The level of technical sophistication allowed for through the use of the monitoring post is higher than the basically ad hoc procedures and hardware now called for by a design engineer, and more importantly, well beyond the equipment readily available to a contractor typically engaged in such site preparation work. For this reason it is believed that this level of control and assurance would only be attainable when designers and contractors have access to monitoring posts which are inexpensive and easily installed and read.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the invention are illustrated, merely by way of example, in the accompanying drawings in which:

FIG. 1 depicts the sequence of events commonly used in current practice.

FIG. 2 depicts the sequence of events advocated herein.

FIG. 3 is an elevation view of a preferred embodiment of the apparatus associated with the sheet piling support anchorage.

FIG. 4 is an elevation view of a preferred embodiment of the apparatus for monitoring the progress of ground treatment by pre-loading.

LIST OF REFERENCE NUMERALS

-   10 sheet piling -   11 compressible strata -   12 competent stratum -   13 pre-load fill -   14 ground surface -   20 adjustable passive anchor assembly -   21 anchor plate -   22 deformable cushion -   23 connector -   24 cushion -   25 turnbuckle or similar device -   30 monitoring post -   31 inner metal pipe -   32 penetration tip -   33 auger -   34 top plate or cap -   35 monitoring leads exit hole -   36 linear variable differential transducer, or similar device -   37 piezometer compartment -   38 outer sleeve -   39 annular plate -   40 interface of pre-load fill and original ground surface -   41 target for differential linear variable differential transducer -   42 sealed bushing

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 3 shows schematically sheet pile sections 10 required for the isolation wall. These are standard construction items, and are available in plastic as well as steel. The installation of the piles is achieved by driving them thorough the compressible strata 11 and into the upper competent stratum 12 prior to placing the pre-load fill 13 on top of the ground surface 14. A variety of pile driving hammers are available; for the circumstances involved in this application it might be preferable to use vibratory, rather than impact (drop or diesel) pile driving hammers. Such pile driving equipment involve no novelty. To make this system function optimally, two pieces of novel apparatus are preferable, and these are as follows:

Description of the Anchor Assembly

FIG. 3 shows schematically adjustable passive anchor assembly 20 which includes the following arrangement of parts: A strong relatively rigid plate 21, preferably rectangular. A deformable space 22 is provided behind, to the side of the plate remote from the wall, to support it in an upright inclined position. The anchor plate is attached near the top of the sheet piling by means of a connector 23 of adjustable length. During the pre-loading period the settlement of the fill overlying the anchor plate will tend to force it into a more reclined position, and this reorientation can be accommodated by the cushion 24. At the same time the connector would be kept suitably taut by means of a turnbuckle 25 or similar device.

Description of Monitoring Post

FIG. 4 shows schematically monitoring post 30, which includes the following arrangement of parts: An inner metal pipe 31 provided with a penetration tip 32 at the bottom end, where this tip might incorporate an auger 33 to facilitate its insertion through the compressible strata 11 and seating into the top of the underlying competent stratum 12. The top of the pipe is covered with a plate or cap 34 to prevent entry of moisture and dust. A hole 35 is provided beneath the cap to allow monitoring leads to exit. This pipe would come in sections which would be coupled together to accommodate various pre-load fill 13 and soil strata thicknesses. One of the sections would contain a linear variable differential transducer 36 (or similar device) capable of measuring the relative vertical movement of a target outside, and adjacent to, the pipe.

The pipe is provided with a compartment 37 near the tip, and possibly others higher up, each to house an electronic piezometer. This compartment is sealed from the interior of the pipe to prevent water entering the pipe and thereby rendering the pore pressure readings questionable. The compartment has a filtered opening (not shown here) to the allow groundwater access to the piezometer's sensitive zone.

An outer sleeve 38 is placed over the inner pipe, extending from the top of the compressible strata to above the top of the pre-load fill. An annular plate 39 is attached to the bottom of this sleeve to rest on the ground surface and thereby allow the settlement of that interface 40 to be measured independently of the tip movements. This sleeve contains the target 41 for the linear variable differential transducer. Bushings with seals 42 are incorporated at either end of the sleeve to minimize friction between these concentric pipes, and to allow slippage while keeping them aligned.

The elevation of the top plate of the monitoring units can then be used to determine the amount of settlement (if any) of the competent bearing stratum. The differential transducer output will measure settlement of the compressible strata. The piezometric output will indicate the rate of progress of consolidation settlement within the compressible layer.

OPERATION OF THE INVENTION

In order to protect adjacent structures from the effects of pre-loading a building site, and to maximize the usable building area, and to ensure that both these aims are achieved in the most effective and timely manner, the contractor would proceed generally as follows:

In the normal fashion, preexisting ground cover and deleterious surficial mineral and organic matter would be removed from the proposed structural footprint and an annular margin around it. In this case however the proposed footprint could be as close to the property line as local building codes allowed, rather than having to provide a no-build zone in anticipation of ground settlement occurring beyond the extent of the pre-loading fill and its side slopes. This approach is illustrated in FIG. 2.

Either after or before this site clearing and grubbing, a sheet pile wall would be installed around the perimeter of the area of land to be preconditioned. This wall could approach to within construction tolerance of the property lines. The piling would be installed (preferably using a vibratory hammer) through the full depth of the compressible strata, and driven a short distance into the underlying competent (essentially non-compressible) stratum. Of course, where the proposed structure was sufficiently remote from adjoining property or adjacent structures, the sheet pile wall would not be installed, as isolation of the ground effects by deformation containment would not be needed.

Sections of the anchor assembly would be erected at ground level and attached to the top of the piling at intervals along the length of the wall. A suitable length of connector, consisting of steel rod, or cable, or chain would be used to attach the top of the piling to the anchors. Also, at this time monitoring posts would be installed by advancing them through the compressible strata and implanting the penetration tip in the competent stratum.

The pre-load fill would then be placed to the design height. Depending on the thickness and type of the fill used, the connectors would be made taut, either during or after the fill placement, in order to provide adequate lateral restraint to the top end of the piling. A turnbuckle or similar device would be used to accomplish this tensioning.

Recorded over time, the data obtained from the linear variable differential transducer and piezometer(s) would provide all the information necessary to determine the behavior of the compressible strata under the influence of the pre-load fill. At the appropriate time the designer would instruct the pre-load fill to be removed.

Periodic measurement of the elevation of the top plate of each monitoring post would indicate any settlement of the underlying competent stratum.

After removal of the pre-load fill, the sheet piling, the anchor assemblies, and the monitoring posts would be removed for return to the equipment renter or for reuse by the specialist subcontractor on the next similar project. Building the structure would be the next activity. 

1. A method involving the use of sheet piling whereby the existing pre-loading technique of ground improvement or treatment can be improved so as to prevent damage to adjacent property and maximize the area of a building site which can be built on without adversely affecting adjacent property.
 2. Apparatus called an anchor assembly which when used in conjunction with sheet piling erected for the temporary containment of fill material can make lateral constraint of the piling more effective by its being adaptable to ground deformations.
 3. An apparatus called a monitoring post which incorporates within itself the means of acquiring all geotechnical information necessary to fully assess the progress of pre-loading, and which can inform the design engineer on demand and without the need for personnel to visit the site. 